For low trading prices, trading on Open Transactions servers might work. They do not even have mechanism in place for taking a percentage of trades at all, the only support they have for fees really is "usage tokens" against which all API calls are charged.

On first glance it seems as if this should help for reasonably-sized trades, but hinder stragegies that want to make many API calls per trade.

I have not checked yet whether the API calls themselves are reasonably "priced" in terms of the usage tokens, such as whether one can get a huge mass of data in one API call instead of having to make one call per market or suchlike.

I have set up my Open Transactions server to use integers for all assets, deliberately to help encourage volume trades. (As to get three digits of price granularity you'd have to use 3 digits of scale, like "100 of this for 101 of that".)

On the other hand I am not a fan of high frequency trading, I have often thought of actually bundling all offers within a half hour or so into one batch clearing run that processes them all at once, specifically to keep bots from butting in between a matching pair of offers to grab most of the difference that otherwise would have gone to the up-front "honest traders" who admitted what they wanted instead of lurking in the background "sniping".